The present invention generally relates to an image display arrangement, and more particularly, to an image display apparatus provided with a refraction factor distribution type flat-plate microlens array, which is suitable as a light condensing or converging device for brightening the display surface by focusing illuminating light for a transmission the display panel such as a liquid crystal panel, etc. having a plurality of picture elements or pixels, onto the region of the pixels.
Commonly, in the liquid crystal display panel to be used in the projection type liquid crystal display apparatus as referred to above, minimum display units called pixels or picture elements (referred to as pixels hereinafter) are regularly arranged. By applying independent driving voltages to the respective pixels so as to vary optical characteristics of the liquid crystal constituting each pixel, images and characters, etc. are displayed as desired.
For impressing independent driving voltages to the respective pixels, a simple matrix system has been known. An active matrix system in which non-linear two terminal elements such as MIM (Metal Insulator Metal) or the like, or three terminal switching elements such as thin film transistors, etc. are provided in the respective pixels.
In the active matrix system, it is required that, in order to apply the independent driving voltage to each of the pixels, elements such as the thin film transistors or MIM, etc. are provided on the respective pixels. Lines for supplying driving signals thereto are connected between the pixels. Therefore, rate occupied by the pixel region in the image surface (or numerical aperture) is reduced. Of the light projected onto the panel, light incident upon a region other than the pixel region is absorbed or reflected by the thin film transistors, signal lines, or shield masks provided depending on necessity, etc., and does not reach a screen. Accordingly, in the case where the liquid crystal panel is illuminated by the same intensity of illumination, the image face becomes darker as the numerical aperture is reduced. Such a state applies both to the cases where the liquid crystal panel is directly observed and also, where enlarged projection is effected by a projection lens.
In order to solve the problem that the image surface becomes dark due to the small numerical perture, there have conventionally been proposed, for example, in Japanese Laid-Open Publications Tokkaisho Nos. 60-165621 to 60-165624, methods for improving utilizing rate of illumination light by condensing it onto each pixel region, with a microlens array being provided at the light source side of the display panel. Particularly, Tokkosho No. 60-165621 discloses formation of a refraction index distribution region on a substrate of the display panel, and illustrates a refraction index distribution diagram having a concave lens effect in its embodiment. Meanwhile, Tokkaisho No. 60-262131 discloses a provision of microlens arrays at opposite sides of the liquid crystal panel.
FIG. 3 shows a refraction index distribution type flat-plate microlens array M which has been conventionally used as a condensing or converging device of projection light onto a liquid crystal panel as referred to above.
The known flat-plate microlens array M in FIG. 3 includes a substrate 101, and a plurality of microlenses 111 formed on the substrate 101 by an ion exchange process, which is a diffusion process, so that the neighboring microlenses may not contact each other. The sectional shape of the refraction index distribution region is formed into approximately a semi-spherical configuration. Accordingly, an advanced line 112 of an ion diffusion region called a diffusion front of the microlenses 111 is separated as shown.
The reason for separating the diffusion front 112 which is the boundary between the respective microlenses 111 as described above, is such that it has been generally considered that rotational symmetry of the refraction distribution configuration with respect to the optical axis, which is necessary to obtain a favorable lens effect, is undesirably impaired by the fusion of the diffusion front 112, thereby producing aberration component with directivity. This gives rise to deterioration in the condensing characteristic of the lens.
However, the conventional flat-plate microlens array as a condensing device referred to above has problems as describe hereinbelow.
The first problem is such that, even when the refraction index distribution of the microlenses 111 is in approximately a semi-spherical shape, rotationally symmetrical with respect to the optical axis, if the distribution thereof in the radial direction is improper, spherical aberration tends to take place. This adversely affects the condensing characteristic of the lens.
With respect to the above, the present inventors have found, through repeated experiments and investigations that, generally in the case where a flat-plate microlens array is produced by the ion exchange method, gradient of the refraction index distribution in the vicinity of the diffusion front 112 is large. Therefore, the optical axis incident upon the surrounding region of the microlenses 111 is excessively refracted, thus resulting in spherical aberration.
The second problem resides in that, since the microlenses 111 are arranged so that the neighboring ones are out of contact with each other, filling rate of the microlenses 111, i.e. ratio of the area occupied by the microlenses 111 to the total area of the microlens array substrate 101, cannot be made higher than a certain limit. By way of example, such limited filling rate may be represented by EQU .pi./4.div.78.5%
when circular microlenses are arranged in a square lattice shape, and by EQU .pi./2.multidot..sqroot.3.div.90.6%
in the case where circular microlenses are densely arranged in a hexagonal lattice shape. Further, in the actual practice, since the microlens arrangement must be in agreement with the pixel arrangement of the display panel, the filling rate becomes even lower than the above. Since gaps among the microlenses are of an ion non-diffusion region and have no refracting function, light incident upon the region advances straight as it is, .without contributing to the light condensing effect.
As described so far, the conventional flat-plate microlens array has problems with respect to the light condensing characteristic. Therefore, in the conventional projection type image display device employing such flat-plate microlens array, darkness in the display image surface has not been fully improved.